Problem: Tiffany did 8 more push-ups than Luis at night. Tiffany did 28 push-ups. How many push-ups did Luis do?
Tiffany did 28 push-ups, and Luis did 8 fewer than that, so find the difference. The difference is $28 - 8$ push-ups. She did $28 - 8 = 20$ push-ups.